1. Field of the Invention
The present invention relates to a magnetic sensor including a magnetoresistive element, and to a magnetic sensor system including the magnetic sensor and a scale.
2. Description of the Related Art
In recent years, magnetic sensor systems have been employed to detect a physical quantity associated with the rotational movement or linear movement of a moving object in a variety of applications. Typically, a magnetic sensor system includes a scale and a magnetic sensor, and the magnetic sensor is configured to generate a signal associated with the relative positional relationship between the scale and the magnetic sensor. Such a magnetic sensor system is disclosed in JP 2008-151759A, WO 2008/072610 A1, US 2003/0137381 A1 and U.S. Pat. No. 5,909,115, for example.
The scale of the magnetic sensor system for use with a rotationally moving object is, in general, a rotating body that moves in response to the movement of the moving object. The rotating body can be, for example, a multipole-magnetized magnet having a plurality of pairs of N and S poles alternately arranged in a circumferential direction, or a gear having teeth formed of a magnetic material. In this case, the magnetic sensor system detects, for example, the rotational position and/or the rotational speed of the rotating body as the physical quantity.
The scale of the magnetic sensor system for use with a linearly moving object is, for example, a linear scale having a plurality of pairs of N and S poles alternately arranged in a linear configuration. In such a case, one of the linear scale and the magnetic sensor moves in response to the movement of the moving object, and the magnetic sensor system detects the relative position and/or speed of the linear scale with respect to the magnetic sensor as the physical quantity.
The magnetic sensor includes a magneto-sensitive element. JP 2008-151759A, WO 2008/072610 A1, and US 2003/0137381 A1 each disclose a magnetic sensor that employs a spin-valve magnetoresistive (MR) element as the magneto-sensitive element. The spin-valve MR element includes a magnetization pinned layer having a fixed magnetization, a free layer having a magnetization whose direction and magnitude vary depending on an external magnetic field, and a nonmagnetic layer interposed between the magnetization pinned layer and the free layer. Examples of spin-valve MR elements include a TMR element in which the nonmagnetic layer is a tunnel barrier layer and a GMR element in which the nonmagnetic layer is a nonmagnetic conductive layer.
In the magnetic sensor employing a spin-valve MR element, the free layer may sometimes receive an interlayer coupling magnetic field resulting from the magnetization pinned layer, as described in JP 2008-151759A. The interlayer coupling magnetic field is in a direction the same as or opposite to the magnetization direction of the magnetization pinned layer. If the free layer receives the interlayer coupling magnetic field, there arises the problem that the amount of a change in the resistance of the MR element with respect to a change in the magnitude of the external magnetic field varies depending on the direction of the external magnetic field, and the problem of a reduced amount of a change in the resistance of the MR element with respect to a change in the magnitude of the external magnetic field.
JP 2008-151759A describes that reducing the interlayer coupling magnetic field to zero makes the output waveform of the magnetic sensor unstable. JP 2008-151759A further teaches a technique for stabilizing the output waveform of the magnetic sensor by applying a bias magnetic field to the free layer in a direction orthogonal to the magnetization direction of the magnetization pinned layer through the use of a permanent magnet. However, when the interlayer coupling magnetic field is not zero, this technique cannot solve the aforementioned problem associated with the interlayer coupling magnetic field.
WO 2008/072610 A1 teaches providing a soft magnetic material element on a side of the MR element so as to apparently improve the magnetic detection sensitivity of the magnetic sensor even in the presence of an interlayer coupling magnetic field being received by the free layer. However, this technique does not directly reduce the effect of the interlayer coupling magnetic field.
US 2003/0137381 A1 and U.S. Pat. No. 5,909,115 each describe a technique for applying a bias magnetic field to the MR element. However, these documents give no consideration to the effect of the interlayer coupling magnetic field.